Blow-molding is a well-known fabrication method for thermoplastic components. The process generally involves the molding of a hollow tube, or "parison," of molten thermoplastic that is lowered from an overhanging extrusion head to a position between halves of a reciprocating mold. As the mold halves close, air or some other gas is injected into the parison; the increased air pressure within the parison caused by such injection forces the parison walls into the contours of the cavities of the mold halves, thus forming the parison into a desired molded shape. The resulting component has molded walls that surround a hollow chamber. Blow-molding has proven to be particularly popular for the production of large parts that would require unduly large molding injection molding machines.
One type of blow-molding that has been used successfully for large components that require structural rigidity is the so-called "double-wall" blow-molding process. In this process, mold halves are most often designed as distinct core and cavity halves (rather than as two cavities, as would be the case for single-wall blow-molded articles, such as bottles or other containers). The core portion of the core mold half extends within the cavity as the mold halves close. In addition, the mold halves for double-wall components are configured so that the molded components have "full-perimeter flash"; i.e., after molding the component has excess material, or "flash", around the perimeter defined by mating surfaces of the mold halves. This contrasts with single-wall components, in which the parison is inflated entirely within closed mold cavities, and the molded component typically has flash only on its top and bottom portions. Double-wall blow-molded components have distinct inner and outer walls that surround a hollow space, with the inner wall having been formed by the core and the outer wall having been formed by the cavity, and with the inner and outer walls being separated by the weld line remaining after the flash is removed. In a typical double-wall component the inner and outer walls are positioned proximate to one another and can have "pinched-off" areas, in which the inner and outer walls are contiguous.
One distinct advantage provided by double-wall blow-molded components is the capability for adjacent regions of the inner and outer walls to differ significantly in their localized contour. For example, a region of the outer wall may have a relatively flat profile, while the adjacent region of the inner wall can contain numerous projections, recesses, and the like, with the profile of either localized region failing to impact significantly the appearance or structural integrity of the other. Such differences in localized inner and outer wall contour are less likely to be successfully achieved in injection-molded components because the inclusion of substantial detail in the inner wall can have a deleterious effect on the dimensional stability, appearance, and even strength of the outer wall. Another performance advantage conveyed by double-wall components stems from the formation of the hollow chamber within the inner and outer walls, as it can provide an air cushion that protects items contacting the inner wall.
For these reasons, double-wall blow-molded components have proven to be particularly popular for protective containers and carrying cases. Detailed contour that mates with, matches, supports, or captures portions of an item to be carried within the carrying case can be included in the inner wall of the double-wall component even as the outer wall has a generally flat, appearance-sensitive surface. Further, the air cushion between the inner and outer walls helps to protect the item. Thus, the container can have the detail and structure necessary to support, transport and protect the item while providing the desired aesthetic appeal, and can do so without the manufacturer having to produce two separate parts for the inner and outer walls.
A typical carrying case includes two components (ordinarily a container and a lid) that are pivotally interconnected along one edge to enable the lid to open and close. It is preferred that much of the structure that forms the hinge for these components be molded into the lid and container. Some hinges employ only structures that are molded into the lid and container (see, for example, U.S. Pat. No. 5,361,456 to Newby, Sr., which employs a molded-in post and receptacle design), while other hinge configurations include one or more additional components.
One popular hinge configuration that includes an additional component besides the lid and container is the "pinned hinge" design, in which an elongate pin is inserted into hollow cylindrical or semicylindrical structures located on the lid and container. These structures of the lid and container hold the pin in place, but are free to rotate about the pin, which in turn allows the lid to pivot relative to the container. Pinned hinge designs generally exhibit good strength, particularly because the material of the pin can differ (and accordingly, can be stronger than) from the material of the lid and container structures. Examples of pinned hinge designs are illustrated in U.S. Pat. No. 4,615,464 to Byrns (which involves a separate step of drilling a hole for the pin after molding), and U.S. Pat. No. 5,208,453 to Rutenbeck et al. (which employs a pin insert that is molded into the hinge during molding).
One issue of pinned hinge designs of the type noted above involves retaining the pin in position. In order to maintain the pin in position, the hinge structures of the lid and container form a slight "interference" fit with the pin. The interference fit between the pin and the hinge structures can undesirably increase frictional resistance to rotation. Also, over long-term use, the plastic forming the hinge structures can "creep" (i.e., slowly flow over time to reduce the hoop stress caused by the interference fit), which can also reduce the ability of the hinge structures to maintain the pin in position. Further, designs that utilize a fully cylindrical structure to capture the pin must either be formed during molding by a "side-action" mechanism or a mold insert (either of which can complicate the molding process and/or increase the cost of the mold), or must be formed in a secondary operation (such as post-molding drilling).